Photolithography is a key component of semiconductor device manufacturing. A photosensitive composition that is often called a photoresist or resist is spin coated on a substrate and is patterned with a mask and an exposure system. The pattern is subsequently transferred into one or more underlying layers by a plasma etch method. Process control of the lithography step is crucial in order to maintain the critical dimension (CD) of features in the printed pattern within a specified range.
Characterizing the optical properties and thickness of the photoresist and underlying layers is necessary to establish CD control. Typically, one or more wavelengths in a range from about 10 nm to about 500 nm is used to expose the photoresist. The light that passes through the photoresist may reflect off underlying layers and pass through the photoresist a second time and can either constructively or destructively interfere with radiation that passes in the opposite direction. Depending on the thickness of the resist, the total energy or accumulated radiation per unit area (referred to as milliJoules/cm2) required to print a feature at a desired CD varies according to the curve shown in FIG. 1 where the plot of thickness vs. exposure dose forms a sinusoidal curve 5 that has minima 1, 3 and maxima 2, 4. This “swing” curve has an amplitude A between a minimum and a maximum energy and a periodicity B defined as the distance (thickness) between two adjacent minimum points or between two adjacent maximum points on the curve 5. The magnitude of periodicity B is related to the wavelength of the exposing radiation. The amplitude A is calculated by dividing the difference between the energy for maximum point 2 (E2) and the energy for minimum point 1 (E1) by the average of E1 and E2 which is (E2−E1)/(E1+E2/2) and this value can be as large as 0.3 which is a swing of 30% in dose. Clearly, an accurate thickness measurement is needed in order to determine that a resist thickness target and thickness uniformity on a wafer is within a tight specification so that a large swing effect can be avoided.
The swing amplitude has a detrimental effect on the lithography process, especially if the magnitude is more than a few % of the average dose. Consider the condition in FIG. 1 where a swing amplitude of 30% is realized as determined previously for E1 and E2 and the patterned feature is a contact hole. If a dose E1 is used to form a contact hole in a photoresist with a thickness T1 on one wafer and the same mask and dose E1 are used to pattern the same photoresist but with a thickness T2 on a second wafer, then the size of the hole with thickness T1 will be much larger than the hole size with thickness T2 since the latter requires a much higher energy to form a hole to a predetermined size. The size difference in space width of the hole is likely to be much greater than the ±10% maximum variation typically allowed for a manufacturing process. Therefore, it is important to keep film thickness uniform within a wafer and from wafer to wafer and to minimize the swing curve to avoid a wide range of CD sizes.
In some situations, an anti-reflective coating (ARC) is applied to the substrate prior to the photoresist coating in order to control reflectivity during the photoresist exposure step and enable a larger process latitude by reducing the swing effect. The ARC which may be an organic or inorganic material is normally much thinner than the photoresist layer and is most effective on relatively flat substrates. However, thickness control for the ARC layer is also important. FIGS. 3a-3c demonstrate how the magnitude of a photoresist swing curve changes as a silicon nitride ARC thickness is varied. The silicon nitride layer has a thickness of 540 Angstroms in FIG. 3a, 750 Angstroms in FIG. 3b, and 1030 Angstroms in FIG. 3c. Reflectivity is minimized by keeping the film thickness of both photoresist and the underlying ARC layer within a tight specification and by tuning the optical constants (n and k) of both layers, if possible. Therefore, it is important to use a method for measuring optical constants n and k that is accurate and reliable and one that requires a minimum of time since several iterations in the photoresist and ARC development process may be necessary before a manufacturable photoresist or ARC product is achieved. In other words, optical film measurements are as important to optimizing a new photoresist or ARC formulation as to controlling a patterning process in manufacturing.
Spectroscopic ellipsometry (SE) is a well accepted tool to extract optical constants n and k in the industry and may be combined with other optical measurement instruments such as a spectrometer and reflectometer. For example, U.S. Pat. Nos. 5,798,837, 6,304,326, 6,411,385, and 6,417,921 assigned to Therma-Wave describe a composite optical measurement system that involves internal calibration as well as n and k measurements of single layer or multi-layer film stacks. Therma-Wave is the only SE tool company that offers an integrated system (Opti-Probe series) with the “expert” level or independent optical thickness measurement components called BPR (Beam Profile Reflectometry) and BPE (Beam Profile Ellipsometry).
A simplified version of a spectroscopic ellipsometry apparatus is shown in FIG. 2. Deep UV and visible sources 10 provide a beam 11 that is polarized by polarizer 12 and focused by a lens 13 onto a wafer or substrate 14 upon which a sample film has been deposited. The wavelength and incident angle of the beam can be varied. The reflected beam 11a passes through a collimating lens 15, a compensator 16 which rotates in this case, an analyzer 17, and into a detector 18. Ellipsometry does not directly measure film thickness or optical constants n (index of refraction) and k (extinction coefficient) but provides up to three parameters, cos Δ, sin Δ, and tan ψ, which are converted to the complex Fresnel reflection coefficients rP and rS. Here the “p” refers to light polarization parallel to the plane of incidence and “s” refers to light polarization perpendicular to the plane of incidence. SE measures the complex ratio rP/rS as a function of wavelength. Coefficients rP and rS contain information on how the magnitude and polarization of the incident light beam are changed by the film on substrate 14 according to the equations: EP,ref=rP×EP,inc and ES,ref=rS×ES,inc where E is the electric field magnitude and “inc” and “ref” refer to incident and reflected light, respectively.
FIG. 6 is a flow chart depicting how the data from integrated optical measurement tools in an Opti-Probe system comprising a broadbeam spectrometer (BB), a spectroscopic ellipsometer (SE) and a beam profile reflectometer (BPR) is used to calculate thickness (t) and n and k values. The BPR component includes a laser that directs a 675 nm beam onto the sample which is then reflected to a BPR analyzer. It should be noted that the three components (BB, SE, BPR) are connected to the same processor so that the data output from each of the three components can be analyzed simultaneously. In step 30, the substrate with the sample film is positioned on a stage within the integrated system. The sample is probed at various wavelengths in step 31 and the data from the three components is weighted in step 32 according to a percentage that may be X % BPR, Y % SE, and Z % BB where X=55, Y=18, and Z=27, for example. An experimental data output is provided in step 33 and collected in the processor.
In step 34, film stack information such as assumed film thickness and approximate n and k values of the film are entered into a modeling program in the processor. The model is exercised (step 35) in the processor to generate a simulated set of data that can be compared to the experimental data obtained in step 33. The fitting of experimental data to modeling data in step 36 may involve several iterations of changing the modeled input until a best fit of modeling data to experimental data is obtained in step 37. Once a best fit is achieved, the program provides values for n, k, and thickness simultaneously. Unfortunately, the accuracy of the n and k values are strongly dependent on the application engineer's experience. A wrong initial guess for the regression fitting in the modeling phase will turn out erroneous n and k values and unreliable mapping signatures. In order to reduce the potential error introduced by modeling a wrong initial setting or by a low instrument signal to noise ratio, a more robust methodology for solving optical constants n and k is required.
One additional application of the index of refraction as described in U.S. Pat. No. 6,057,928 is that it can be used to determine a dielectric constant. The index of refraction is measured by impinging a high frequency beam on a sample and monitoring the phase change and field reflectance for a variety of incident angles.